1. Field of the Invention
The invention is directed to machinery used for raising loads which include a boom and supporting structure. Examples of such machines include draglines and large industrial cranes.
2. Discussion of Prior Art
A walking dragline typically includes a main housing, a boom which extends upwardly and outwardly from the main housing, and a mast assembly which extends upwardly from the main housing for supporting the boom. The boom comprises a welded frame including longitudinally extending upper and lower pairs of chords and lacing welded between the chords. Support lines extend between the upper end of the mast assembly and the boom for supporting the boom relative to the mast assembly. A sheave mounted for rotation about a horizontal axis is on the upper end of the boom. Hoist ropes extend from a bucket hoist mechanism, over the sheave and then to a bucket. Extending or retracting the hoist ropes causes vertical movement of the bucket. A drag rope extends from a bucket drag mechanism to the bucket to permit horizontal movement of the bucket relative to the boom. The main housing is supported by a tub that sits on the ground when the dragline is engaged in digging operations. The main housing is supported for pivotal movement relative to the tub in a lateral plane about a vertical axis. A pair of walking mechanisms are mounted on opposite sides of the main housing and are operable for moving the main housing over the ground between digging operations. The main housing includes an operator's cab adjacent to and generally beneath the boom. A stairway extends from the inner end to the outer end of the boom.
Because the hoisting rope or ropes are quite heavy by virtue of their own dead weight, when these ropes become slack due to little or no load on them, the ropes can hit the top of the boom. To prevent this from happening, a rope support sheave is provided partway between the lower end and the upper end of the boom in order to prevent the hoist rope from hitting the boom.
When the hoisting rope is taut or under load and extends between the boom top sheave and the hoisting drum, the hoisting ropes are a substantial distance above the boom. As a result, the rope support sheave must be mounted a substantial distance above the boom. In the prior art, a rigid member structure was used to support the support sheave. This added substantial weight to the boom thus reducing the amount of weight which the boom could support. The rope support sheave requires regular maintenance due to the number of mechanical components involved with the sheave. In the prior art, additional access means such as ladders, stairs, platforms, etc. must be added to the boom in order to permit access to the rope support sheave for maintenance, which adds further weight to the boom, thus reducing the weight the boom can carry.